1. Field of the Invention
The present invention relates to the preparation of an alkali metal chlorate or perchlorate by electrolysis, in an aqueous medium, of either the corresponding alkali metal chloride or chlorate, respectively.
2. Description of the Prior Art
It is known to this art to conduct the above electrolysis in an electrolytic cell designed and equipped in a manner suitable for the electrolysis of sodium chloride into chlorine and sodium hydroxide, designated a "chlorine-soda" electrolysis, but in which no porous barrier, diaphragm or membrane is disposed intermediately between the cathode and the anode, such that the cell is not divided into a cathode and an anode compartment.
In the description that follows, the preparation of an alkali metal chlorate is described. However, such description, including the description of the invention, is also applicable to the preparation of an alkali metal perchlorate.
The aforementioned prior art process, in actual practice, is typically carried out in the presence of hexavalent chromium values, sodium chromate or sodium bichromate in order to limit the deleterious cathodic reduction of hypochlorite and/or chlorate ions. The disadvantages of chromium, both relative to process economics as well as from a standpoint of environmental concerns, are set forth, for example, in U.S. Pat. No. 4,295,951. The cathode described in this '951 patent for the conventional electrolysis of chloride into chlorate in a noncompartmentalized cell makes it possible to reduce, but not to completely eliminate the parasitic cathode reactions. The concomitant loss of electrical energy, even if on a reduced scale, clearly presents an economic disadvantage.
Processes have also been proposed to this art for the electrolytic preparation of chlorate from chloride, using a compartmentalized cell designed and equipped in a manner similar to that of a "chlorine-soda" electrolysis cell.
For example, in the processes described in FR 1,502,519, BE 690,501 and U.S. Pat. No. 3,878,072, the chlorine and the alkali metal hydroxide formed in the compartmentalized cell react with each other outside the anode and the cathode compartments to produce the chlorate, the formation of which is avoided in the electrolysis proper.
Again for example, in the process described in FR 2,249,973, the production of chlorate requires a plurality of electrolytic cells, a first compartmentalized electrolytic cell for producing gaseous chlorine and an aqueous solution of an alkali metal hydroxide, as well as a second noncompartmentalized cell which employs the anolyte of the first cell; this plurality of compartments is critical for the production of chlorate.
As indicated above and as is generally well known to this art, the use of hexavalent chromium in the electrolysis of chloride into chlorate in a noncompartmented cell, such as the second cell of the process described in the aforesaid FR 2,249,973, is a necessary evil if an acceptable electrolytic yield is to be attained. But chromium values, together with the disadvantages and drawbacks thereof, are also present in other processes, such as those described, for example, in FR 1,502,519 and BE 690,501, which entail the use of a compartmented cell.
The process described in FR 2,249,978 presents the additional disadvantage of requiring that the chlorate produced in the second cell be treated to separate the chloride, prior to isolation of the chlorate. This disadvantage also exists in the process described in U.S. Pat. No. 3,878,072, wherein, in order to insure a satisfactory production of chlorate, the concentration of the chloride in the anolyte is increased.
Finally, in none of the known processes for the preparation of chlorate using a compartmented cell is the production of only chlorate assured.